Process for removing solvent and nitrocellulose residues from polyolefin shreds

ABSTRACT

Processes are described for the removal of solvent and/or nitrocellulose residues from precleaned polyolefin, in particular from precleaned polyethylene recyclate shreds, whereby the polyolefin is either mixed with washing water, the solvent is expelled and the nitrocellulose is at the same time washed from the polyolefin, or the polyolefin is washed with cleaned solvent, the solvent is mechanically separated, and the solvent residues remaining on the polyolefin are expelled by a stream of nitrogen.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for cleaning polyolefin, inparticular polyethylene recyclate shreds, as well as to devices forcarrying out the process.

2. The Prior Art

Polyethylene is a polyolefin, which is processed in large quantities forthe manufacture of foils for packaging purposes. For example, shrinkhoods, bags and the like are manufactured from the foils.

Polyethylene, as a thermoplastic material, is fusible and formablerepeatedly because of its molecular structure. It is basically suitablefor being reused several times. However, reprocessing to high-qualityrecycling products, preferably to polyethylene foils with definedproperties, is opposed by the fact that the used articles, particularlyfoils, are equipped during their manufacture with application-orientedadditives such as antistatic agents, lubricants, thermal orUV-stabilizers, which are then present in unknown amounts andcombinations in the recycling product manufactured from such articles.Sorting of the used articles according to ingredient substances ishardly possible technically and economically speaking.

Recycling products, in particular foils loaded with such ingredientsubstances lead to problems during further processing, for exampleduring gluing, welding or imprinting. Often, the connecting seams, whichinitially show good properties, come apart only at the user.

Furthermore, the used foils contain degradation products of thepolyethylene such as polyethylene waxes, as well as decompositionproducts of the additives. Such products develop during the life span inthe course of processing processes, as well as due to the action ofheat, light and weathering. They effect to some extent a considerablereduction of the properties of use of the recycling product.

Finally, the used foils, in particular packaging foils, are more or lessheavily imprinted. During the extrusion of recycled materials, suchprinting inks lead to polyethylene foils with a strong undefineddiscoloration, so that only refuse bags dyed in dark colors can bemanufactured from such materials. With very high proportions ofimprinted areas in relation to the amount of polymer, even unusableregranulates foamed by decomposition gases can be obtained.

For removing the foil additives, the nonpolar polyethylene waxes and theadhering printing inks from the used foils, DE-OS 4 122 277 discloses aknown method. It specifies a process in which the used foils areshredded and, under intensive motion, subjected to an extraction in asolvent bath containing an organic solvent.

The shreds treated under heat with an organic solvent, followingmechanical separation of the solvent, still contain about 50 to 60% byweight solvent, for example in the case of ethyl acetate. The solventcan be found both on the surfaces of the shreds and diffused into theinterior of the shreds. In addition, on the surface, the shreds containresidues of nitrocellulose, the preferred vehicle for flexographicprinting inks. Such residual amounts of nitrocellulose, even though low,lead to carbonization during regranulation, and thus to black spots inthe recyclate foil.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide processes by whichpolyolefin, preferably in the form of polyethylene shreds, can becleaned.

According to the invention, this object is achieved by a process for theremoval of solvent and/or nitrocellulose residues from precleanedpolyolefin, particularly from precleaned polyethylene recyclate shreds,said process being characterized in that the polyolefin is mixed withwater, the solvent is either expelled or separated by extraction, andthe residues of nitrocellulose are at the same time washed off from thesurfaces of the polyolefin shreds.

In the process according to the invention, expelling and/or extractioncan take place continuously or discontinuously.

In a special implementation of the process, ethyl acetate is used as thesolvent.

In a discontinuous procedure, for example in an evaporator, the solvent,preferably ethyl acetate, is expelled or evaporated from the surface ofthe polyolefin under stirring at a temperature above the boilingtemperature of the solvent.

In the case of ethyl acetate, 30 minutes stirring at a temperature of85° to 90° C. suffices.

A weight ratio of polyolefin shreds to washing water of 1:8 to 1:10 wasfound to be advantageous.

In the continuous mode of operation, the polyolefin shreds are freedfrom the solvent by means of water slightly below the boilingtemperature of the solvent, preferably at a temperature of 70° to 75° C.A multi-chamber extractor was found to be suitable for this process. Inthe latter, the water passes through individual chambers filled withshreds as a concretely adjustable, continuous volume flow over a definedperiod of time. The water is guided countercurrently to the polyolefinshreds.

After said washing and extracting process, the polyolefin shreds areagain flashed for 5 to 10 minutes with water of high turbulence. Therinsing water can be circulated through an integrated filter forcleaning the water. The rinsing action still removes nitrocelluloseresidues from the surfaces of the polyolefin shreds and separates finestmineral impurities which have been carried along up to this point.

In a special form of implementation of the invention, the solvent loadedwith printing ink pigments and vehicles, foil ingredients as well aslubricants and antistatic agents, said solvent having been separatedfrom the polyolefin shreds mechanically prior to the treatment withwater, is passed into a precipitation vessel downstream.

The precipitation for the quantitative separation of the flexographicprinting ink vehicle nitrocellulose from the solvent can take place withthe washing water. Advantageously, the precipitation takes place understirring at a temperature of 30° to 70° C. and with a precipitationduration of 10 minutes. Preferred is a temperature of 30° to 40° C. anda weight ratio of solvent to water of 1:1.

In this precipitation process, the water-insoluble printing ink pigmentsare completely precipitated at the same time together with thenitrocellulose.

The solvent vapors forming during the washing process are condensed,collected and passed into a solvent tank. This tank serves as the supplytank for the solvent, from where the solvent can be used again in thecleaning and extracting process as a circulation solvent.

The separation of a three-phase mixture solvent/solids precipitate/waterforming during the precipitation can take place in a phase separationbottle.

The precipitate of solids from the three-phase mixture can be separatedby filtration, and the water can be used again as circulation washingwater, whereby the water can be decolored, if need be, via a column withactive carbon.

The main portion of solvent in the precipitate of solids is stirred fora certain time with hot water of about 90° C. in a vessel connecteddownstream, where the solvent is expelled. Furthermore, for completelyremoving the solvent from the precipitate of solids, stripping can becarried out solvent-free with steam to make the solids solvent-free, andthe solvent can be reused following condensation. The precipitateagglomerates and can be separated mechanically without problems. Thesolvent is condensed and can be reused as circulation solvent. In thisway, the solids precipitate is discharged water-moist and free ofsolvent and thus satisfies the safety regulations for transport anddisposal with respect to nitrocellulose.

The solvent-enriched water from the phase separation can be completelyused again for the precipitation of the nitrocellulose.

The solvent from the phase separation has been previously cleaned. Thusa partial stream of this solvent can be recirculated and used as thesolvent in the cleaning step. For example, it may be used for theseparation of the printing inks. This partial stream can be controlledvia a suitable measuring analysis of the solvent after the phaseseparation. For example, measurement of the refractive index or of thedepth of the color is suitable for the measuring analysis.

The other part of the solvent is passed as a partial stream into anevaporator plant for treatment and/or purified via an adsorber column.In this way, this part of the solvent can be completely reused forextraction by recirculation from the solvent supply tank.

This procedure permits a reduction of the amount of solvent required perunit of quantity polyolefin shreds to a technically required minimumamount, whereby the full efficiency of the cleaning and extractingprocess is assured. At the same time, the evaporator output required perunit of quantity polyolefin shreds, i.e., the energy cost for theevaporation is reduced to a minimum.

During evaporation, a residue is collected with the main componentspolyethylene waxes and smaller proportions of higher fatty acids, fattyacid derivatives, phthalates and lacquered printing ink pigments. Theseresidues can be reused as valuable substances for selected applications,for example for the manufacture of certain foil assortments such as, forexample, dyed refuse bags.

According to a second process variation, complete cleaning of thepartial stream of solvent from the phase separation is accomplished bysorption. The attendant substances such as phthalates, polyethylenewaxes and small proportions of printing ink pigments still remaining inthe solvent after the precipitation with water are separated. So-calledadsorber earths such as, for example, magnesium-aluminum hydrosilicatesare suitable for such a process. Special active carbons as well ascombinations of the two adsorbents are suitable as well. The completeomission of an evaporator plant is advantageous with this processvariation; however, the increased costs for the disposal of both theloaded adsorber substance and attendant substances in the solvent aredisadvantageous.

The two partial streams can be jointly admitted into the solvent tankand thus completely reused for a new cleaning and extracting process.

Another variation makes provision for the cleaning of the entire loadedsolvent following cleaning and extracting without water precipitation,by high-efficiency filtration, which can be carried out in combinationwith adsorption or evaporation.

An example of operation showing additional inventive features isschematically shown in FIG. 1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flow diagram explaining the course of the processaccording to the invention.

FIG. 2 shows a second embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1, the dashed lines show the flow of the water; the thin,through-extending lines show the flow of the solvent; and the doublelines the flow of solids.

Used polyolefin shreds in ethyl acetate as the solvent are loaded in acleaning and extracting reactor 1, which also can have the function of asolvent bath.

As soon as the cleaning and extraction is completed, ethyl acetateloaded with foil ingredients, printing ink pigments and printing inkvehicles as well as polyethylene waxes is mechanically separated fromthe polyolefin shreds, and passed into a downstream precipitation vessel2, which is equipped with an agitator as well.

The shreds are suspended with water of 85° to 90° C. and washed understrong motion. It is important in this connection that the temperatureof the water is above the boiling point of the ethyl acetate, so thatthe latter is evaporated.

The liberated solvent vapors can be passed into a solvent tank via acondensation line downstream.

Subsequently, the solvent-free shreds are mechanically separated fromthe washing water.

The washing water, which is loaded with residues of nitrocellulose andethyl acetate, can be fed into the precipitation vessel 2 as well inorder to precipitate the nitrocellulose. The precipitation is carriedout at a temperature of 30° to 40° C., with a precipitation duration of10 minutes and with a ethyl acetate/water ratio of 1:1. From theprecipitation vessel 2, the mixture of ethyl acetate/solidsprecipitate/water is passed into a phase separation bottle 3, in whichthe mixture is separated into the individual phases.

The solids are separated from the mixture by a filtration device 5 andsubsequently stripped solvent-free with steam. Via a partial dehydrationdevice 6, which operates mechanically or thermally downstream, thesolids precipitate can be adjusted to the residual moisture required forsafety reasons with respect to the nitrocellulose.

Part of the water is fed into the precipitation vessel as circulationwater. The other part can be decolored, if necessary, via a column withactive carbon 4. The water so cleaned is now recycled again into thecleaning and extracting plant as circulation washing water in order tofree the polyolefin shreds from the residual solvent and nitrocelluloseresidues.

The solvent from the phase separation is fed into a buffer vessel 7 andsubsequently analyzed in an analyzer device 8.

A partial stream can be reused directly as circulation solvent for theprocess stage of cleaning. The other part of the solvent is passed intoan evaporator plant 9 or adsorber 10, where it is completely cleaned.The cleaned solvent is subsequently collected in a supply tank 11 andused as circulation solvent in the process stage of extraction.

The solvent vapors exiting from the reactor 1 are condensed by thecondenser 12 and the solvent is passed into a supply tank 13. Fromthere, the solvent can be used again as circulation solvent for thecleaning in the reactor 1.

According to the invention, the problem specified above is solved,furthermore, by a process for the cleaning of polyolefin, particularlyof old polyethylene foil shreds, said process being characterized inthat the polyolefin shreds are freed from loose impurities via amechanical dry cleaning, and mixed in the process step of cleaning witha mixture of solvent and water, whereby the three-phase mixture isalready formed in the reactor, which mixture can be subsequently passeddirectly into the phase separation bottle.

In said process, the polyolefin shreds with the residual dirt portionrigidly adhering to said shreds are subjected to cleaning and extractionwith the solvent without prior washing with water. The dirt particlesintroduced in the process are found to be advantageous for theprecipitation of the nitrocellulose with water because they promote theformation of condensation nuclei. This considerably enhances thefilterability of the solids precipitate.

Fine dirt particles carried along through the process steps of cleaningand extraction are treated in a rinsing process with clean water afterthe residual solvent has been removed from the shreds.

A filter for separating the solid particles is installed in the rinsingwater circulation. The duration of the rinsing operation can becontrolled via a measuring system, measuring, for example the degree ofturbidity. It is particularly advantageous with said process of theinvention that the polyolefin shreds are no longer cleaned from dirt viacostly washing with water. Therefore, a reduction of the operating costcan be obtained with said variation of the process of the invention.

According to another process variation for the cleaning of polyolefin,particularly of old polyethylene foil shreds, the polyolefin shredsafter a mechanical dry cleaning, are subjected to a first short cleaningstep with a solvent/water mixture using high frictional motion.

Preferably, the ratio of solvent-to-water comes to 1:1 to 1:3; thereaction time maximally amounts to 5 minutes at a temperature of 50° to60° C. The three-phase mixture already forming in said process step canbe directly discharged into the phase separation bottle. Said variationaccording to the invention has the advantage that cleaning of thepolyolefin shreds via washing with water is dispensed with. Furthermore,the residual dirt rigidly adhering to the shreds is already completelyseparated after the mechanical dry cleaning process step of cleaning.

Furthermore, the problem specified above is solved according to theinvention by a process for the removal of solvent and/or nitrocelluloseresidues from precleaned polyolefin, in particular from precleanedpolyethylene recyclate shreds, said process being characterized in thatthe polyolefin is washed with cleaned solvent, the solvent ismechanically separated, and the solvent residues remaining on thepolyolefin are expelled by a stream of nitrogen.

Ethyl acetate is preferred as the solvent.

Shredded polyethylene foils which have been precleaned by extractionprocesses can be used as the polyolefin.

The polyolefin, which, for example, has been treated for a period of 30minutes under heat with ethyl acetate, still carries about 50 to 60% byweight ethyl acetate after it has been mechanically separated from theethyl acetate.

According to a preferred implementation of the invention, the solventcarried along is expelled by a nitrogen stream of about 85° to 90° C.

The solvent carried along in the stream of nitrogen, followingseparation of the solvent from the nitrogen and intermediate storage ina solvent supply vessel, can be used again as circulation solvent.

According to another advantageous implementation of the invention, thenitrogen freed from the solvent is heated via a heat exchanger to 70° to90° C. and used again as circulation nitrogen for removing the solventfrom the polyolefin.

According to another advantageous development of the process, themechanically separated washing solvent--which is loaded with foilingredients such as lubricants, antistatic agents and polyethylenewaxes, is passed via an adsorber column, in which adsorber earths of themagnesium-aluminum hydrosilicate type can be used as adsorber substancesin combination with active carbon. In this way, ethyl acetate is almostcompletely separated from all attendant substances. The solvent sodecolored and cleaned can be collected in the solvent supply vessel andalso used again as circulation solvent for further cleaning andextraction processes.

An example of operation showing additional features according to theinvention is schematically shown in FIG. 2.

FIG. 2 shows a flow diagram explaining the course of said processaccording to the invention.

For the removal of nitrocellulose residues from precleaned polyethylenerecyclate shreds, cleaned ethyl acetate is loaded in a cleaning vessel14. The washing process is supported by an agitator device in thecleaning vessel 14. The solvent is mechanically separated and cleanedvia an adsorber column 15 downstream, with a solvent container 16 beingconnected downstream of said column.

The ethyl acetate residues remaining on the polyethylene are expelled bya stream of nitrogen having a temperature of 85° to 90° C. The ethylacetate carried along in the stream of nitrogen is freed from thenitrogen by a cold set 17 and used again as circulation ethyl acetatevia the solvent container 16.

The nitrogen freed from the ethyl acetate is heated via a heat exchanger18 and used as circulation nitrogen.

I claim:
 1. Process for removing solvent and nitrocellulose residuesfrom precleaned polyolefin shreds comprising precleaned polyethylenerecyclate shreds, comprising the steps ofmixing the polyolefin shredswith washing water, expelling the solvent by heating the water to atemperature above the boiling temperature of the solvent; washing thenitrocellulose residues from each surface of the polyolefin shreds,using the washing water for precipitating nitrocellulose from thesolvent; and collecting solvent in the form of vapors during washing andpassing said vapors into a solvent tank and, using the collected solventas circulation solvent.
 2. Process according to claim 1,wherein theexpelling takes place continuously discontinuously.
 3. Process accordingto claim 1,wherein ethyl acetate is used as the solvent.
 4. Processaccording to claim 3, comprisingheating the water under stirring to atemperature of 85° to 90° C.
 5. Process according to claim 1,wherein theweight ratio of polyolefin shreds to washing water ranges from 1:8 to1:10, and time of mixing is 30 minutes.
 6. Process according to claim1,wherein the duration of precipitation is 10 minutes and the at atemperature ranges from 30° C. to 70° C.
 7. Process according to claim1,wherein the temperature in the preparation step ranges from 30° C. to40° C. and the weight ratio of solvent to water is 1:1.
 8. Processaccording to claim 1, further comprising the step ofseparating theprecipitate which comprises a mixture of solvent/water/solidsprecipitate into solvent, water and solids precipitate by means of aphase separation bottle.
 9. Process according to claim 8, furthercomprising the steps ofseparating a solids precipitate from thethree-phase mixture by filtration and using the water, as circulationwashing water, whereby the water is first decolored, via a column withactive carbon.
 10. Process according to claim 9, further comprising thesteps ofstripping the solids precipitate to be solvent-free with steamand reusing the stripped solvent following condensation.
 11. Processaccording to claim 1, comprisingcleaning the collected solvent byprecipitation with water to such a degree that following phaseseparation, it can be directly used in a first partial stream ascirculation solvent in a cleaning process step.
 12. Process according toclaim 11, comprisingcleaning the collected solvent in a second partialstream via an evaporator plant and/or an adsorber column.
 13. Processfor removing solvent and nitrocellulose residues from precleanedpolyolefin, comprising the steps ofwashing the polyolefin with cleanedethyl acetate as solvent; mechanically separating the solvent; andexpelling remaining solvent residues from the polyolefin by a nitrogenstream.
 14. Process according to claim 13,wherein the nitrogen streamhas a temperature of 85° to 90° C.
 15. Process according to claim 13,comprisingfreeing the solvent carried along in the nitrogen stream fromnitrogen via a cold set and, using the freed solvent again via a solventcontainer as circulation solvent.
 16. Process according to claim 15,further comprising the steps ofheating nitrogen freed from solvent via aheat exchanger to 70° C. to 90° C. and using the nitrogen again. 17.Process according to claim 13, comprisingpassing the mechanicallyseparated washing solvent via an adsorber column and cleaning saidsolvent in said column, and using said solvent again via a solventcontainer.
 18. Process according to claim 17, comprisingusingmagnesium-aluminum silicate as an adsorber in the adsorber column incombination with active carbon.
 19. Process for removing solvent andnitrocellulose residues from precleaned polyolefin shreds comprising thesteps ofmixing the polyolefin shreds with washing water; expelling thesolvent by heating the water to a temperature above the boilingtemperature of the solvent; washing the nitrocellulose residues fromeach surface of the polyolefin shreds; and using the washing water forprecipitating nitrocellulose from the solvent.